HP 5500 Ei 5500 Si Switch Series Configuration Guide
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• To configure an area as a totally stub area, the stub c o m m a n d mus t b e c o n fig u re d o n ro u te rs i n t he
area, and the ABR of the area must be configured with the stub [ no-summary ] command.
• A totally stub area cannot have an ASBR because AS external routes cannot be distributed into the
stub area.
• Virtual links cannot transit totally stub areas.
NSSA area
Similar to a stub area, an NSSA area does not impo rt AS external LSAs (Type-5 LSAs), but can import...](http://img.usermanuals.tech/thumb/36/58909/w64_hp-5500-ei-5500-si-switch-series-configuration-guide-747.png "Page 748
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• To configure an area as a totally stub area, the stub c o m m a n d mus t b e c o n fig u re d o n ro u te rs i n t he
area, and the ABR of the area must be configured with the stub [ no-summary ] command.
• A totally stub area cannot have an ASBR because AS external routes cannot be distributed into the
stub area.
• Virtual links cannot transit totally stub areas.
NSSA area
Similar to a stub area, an NSSA area does not impo rt AS external LSAs (Type-5 LSAs), but can import...")
62 DR and BDR Introduction On a broadcast or NBMA network, any two routers need to establish an adjacency to exchange routing information with each other. If n routers are present on the network, n(n-1)/2 adjacencies are required. In addition, any topology change on the network results in traffic for route synchronization, which consumes many system and bandwidth resources. The Designated Router (DR) was introduced to solve this problem. On a network, a DR is elected to advertise routing information among other routers. If the DR fails, routers on the network have to elect another DR and synchronize information with the new DR. It is time-consuming and pron e to routing calculation errors. The Backup Designated Router (BDR) can solve this problem. The BDR is elected along with the DR and establishe s adjacencies with all other routers. When the DR fails, the BDR becomes the new DR in a very short time. Meanwhile, other routers elect a new BDR. Routers other than the DR and BDR are called DRothers. They do not establish adjacencies with one another. Thus the number of adjacencies is reduced. In Figure 23 , s olid lines are Ethernet physical links, and dashed lines represent OSPF adjacencies. In the network with the DR and BDR, only seven adjacencies are needed. Figure 23 DR and BDR in a network DR and BDR election Routers in a network elect the DR and BDR according to their router priorities and router IDs. Routers with a router priority value higher than 0 are candidates for DR/BDR election. The election votes are hello packets. Each router sends the DR elected by itself in a hello packet to all the other routers. If two routers on the network declare themselves as the DR, the router with the higher router priority wins. If router priorities are the same, the router with the higher router ID wins. In addition, a router with router priority 0 cannot become the DR or BDR. • DR election is available on broadcast and NBMA interfaces rather than P2P and P2MP interfaces. • A DR is an interface of a router and belongs to a single network segment. Another interface of the router may be a BDR or DRother. • If a router with the highest router priority is adde d after DR/BDR election, the router cannot become the DR immediately. • The DR may not be the router with the highest priority in a network, and the BDR may not be the router with the second highest priority. DR BDR DRother DRother DRother
63 OSPF packet formats OSPF packets are directly encapsulated into IP packets. O S P F u s e s t h e I P p r o t o c o l n u m b e r 89. T h e f o r m a t of an OSPF LSU packet is shown in Figure 24. Figure 24 OSPF packet format OSPF packet header OSPF packets are classified into five types that have the same packet header. Figure 25 OSPF packet header Major fields of the OSPF packet header are as follows: • Version —OSPF version number, which is 2 for OSPFv2. • Ty p e —OSPF packet type from 1 to 5, corresponding to hello, DD, LSR, LSU, and LSAck, respectively. • Pac ke t l e ngt h —Total length of the OSPF packet in bytes, including the header. • Router ID —ID of the advertising router. • Area ID —ID of the area where the advertising router resides. • Checksum—Checksum of the message. • AuType —Authentication type, ranging from 0 to 2, corresponding to non-authentication, simple (plaintext) authentication, and MD5 authentication, respectively. • Authentication —Information determined by authentication type. It is not defined for authentication type 0. It is defined as password information for authentication type 1, and defined as Key ID, MD5 authentication data length, and sequence number for authentication type 2. NOTE: MD5 authentication data is added followin g an OSPF packet rather than contained in the Authentication field. Hello packet A router sends hello packets periodically to find and maintain neighbor relationships, and to elect the DR or BDR, including information about values of timers, DR, BDR, and neighbors that are already known.
64
Figure 26 Hello packet format
Major fields of the hello packet are as follows:
• Network mask —Network mask associated with the router’s sending interface. If two routers have
different network masks, they cannot become neighbors.
• HelloInterval —Interval for sending hello packets. If two routers have different intervals, they cannot
become neighbors.
• Rtr Pri —Router priority. A value of 0 means the router cannot become the DR or BDR.
• RouterDeadInterval —Time before declaring a silent router down. If two routers have different dead
intervals, they cannot become neighbors.
• Designated router —IP address of the DR.
• Backup designated router —IP address of the BDR.
• Neighbor —Router ID of the neighbor router.
DD packet
Two routers exchange database description (DD) packets, describing their LSDBs for database
synchronization. A DD packet contains only the headers of LSAs to reduce traffic.
...
Network mask
HelloInterval Options Rtr Pri
RouterDeadInterval
Designated router
Backup designated router
Neighbor
Version1
Router ID
Area ID
Checksum AuType
Packet length
Authentication
Authentication
0715 31
Neighbor
65 Figure 27 DD packet format Major fields of the DD packets are as follows: • Interface MTU —Specifies the largest IP datagram in by tes that the interface can send without fragmentation. • I (Initial)—The Init bit, which is set to 1 if the packet is the first DD packet. It is set to 0 if not. • M (More) — Th e M o re bi t, wh ich i s s e t t o 0 i f t h e p a cke t i s t h e l as t D D p a cke t. I t i s s e t to 1 i f m o re D D packets are to follow. • MS (Master/Slave) —The Master/Slave bit. When set to 1, it indicates that the router is the master during the database exchange process; otherwise, the router is the slave router. • DD sequence number —Used to sequence the collection of DD packets. The initial value is set by the master. The DD sequence number then increments until the complete database description has been sent. LSR packet After exchanging DD packets, two routers know which LSAs of the peer are missing from the local LSDB. Then, they send (link state request) LSR packets to request the missing LSAs. An LSR packet contains the brief of the missing LSAs. ...
66
Figure 28 LSR packet format
Major fields of the LSR packets are as follows:
• LS type —Type of the LSA to be requested. Type 1 for example indicates the Router LSA.
• Link state ID —Determined by LSA type.
• Advertising router —ID of the router that sent the LSA.
LSU packet
LSU (Link State Update) packets are used to send the requested LSAs to the peer. Each packet carries a
collection of LSAs.
Figure 29 LSU packet format
LSAck packet
Link State Acknowledgment (LSAck) packets are used to acknowledge received LSU packets. An LSAack
packet carries the headers of LSAs to be acknowledged.
Version3
Router ID
Area ID
Checksum AuType
Packet length
Authentication
Authentication
LS type
Link state ID
...
Advertising router
0715 31
...
67 Figure 30 LSAck packet format LSA header format All LSAs have the same header. Figure 31 LSA header format Major fields of the LSA header are as follows: • LS age —Time, in seconds, elapsed since the LSA was originated. An LSA ages in the LSDB (added by 1 per second), but does not age during transmission. • LS type —Type of the LSA. • Link state ID —The contents of this field depend on the LSAs type. • LS sequence number —Used by other routers to judge new and old LSAs. • LS checksum —Checksum of the LSA except the LS age field. • Length —Length in bytes of the LSA, including the LSA header. LSAs formats • Router LSA ...
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Figure 32 Router LSA format
Major fields of the Router LSA are as follows:
{ Link state ID —ID of the router that originated the LSA.
{ V (Virtual Link) —Set to 1 if the router that originated the LSA is a virtual link endpoint.
{ E (External) —Set to 1 if the router that originated the LSA is an ASBR.
{ B (Border) —Set to 1 if the router that originated the LSA is an ABR.
{ # Links —Number of router links (interfaces) to the area, as described in the LSA.
{ Link ID —Determined by link type.
{ Link data —Determined by link type.
{ Ty p e —Link type. A value of 1 indicates a point-to-point link to a remote router; a value of 2
indicates a link to a transit network; a value of 3 indicates a link to a stub network; and a value
of 4 indicates a virtual link.
{ #TOS —Number of different TOS metrics given for this link. If no TOS metric is given for the link,
this field is set to 0. TOS is not supported in RFC 2328. The #TOS field is reserved for early
versions of OSPF.
{ Metric —Cost of using this router link.
{ TOS —IP Type of Service that this metric refers to.
{ TOS metric —TOS-specific metric information.
• Network LSA
A Network LSA is originated by the DR on a br oadcast or NBMA network. The LSA describes all
routers attached to the network.
...
0# Links
Link ID
Link data
Type
TOS
Link ID
Link data
...
V0EB
#TOSMetric
0TOS metric
LS age
Link state ID
Advertising router
Options1
LS sequence number
LS checksumLength
071
531
69
Figure 33 Network LSA format
Major fields of the Network LSA are as follows:
{ Link state ID —The interface address of the DR.
{ Network mask —The mask of the network (a broadcast or NBMA network).
{ Attached router —The IDs of the routers, which are adjacent to the DR, including the DR itself.
• Summary LSA
Network summary LSAs (Type-3 LSAs) and ASBR su mmary LSAs (Type-4 LSAs) are originated by
ABRs. Except for the Link state ID field, the fo rmats of Type 3 and 4 summary-LSAs are identical.
Figure 34 Summary LSA format
Major fields of the Summary LSA are as follows:
{ Link state ID —For a Type-3 LSA, it is an IP address outside the area. For a type 4 LSA, it is the
router ID of an ASBR outside the area.
{ Network mask —The network mask for the type 3 LSA. It is set to 0.0.0.0 for the Type-4 LSA.
{ Metric —The metric to the destination.
NOTE:
A Type-3 LSA can be used to advertise a default rout e if the link state ID and network mask are set to
0.0.0.0.
• AS external LSA
70
An AS external LSA is originated by an ASBR, and describes routing information to a destination
outside the AS.
Figure 35 AS external LSA format
Major fields of the AS external LSA are as follows:
{ Link state ID —The IP address of another AS to be advertised. When describing a default route,
the Link state ID is always set to default destination (0.0.0.0) and the network mask is set to
0.0.0.0
{ Network mask —The IP address mask for the advertised destination
{ E (External Metric) —The type of the external metric value, which is set to 1 for type 2 external
routes, and set to 0 for type 1 external routes. See Route types
for a description of external
route types.
{ Metric —The metric to the destination.
{ Forwarding address —Data traffic for the advertised destination is forwarded to this address.
{ External route tag —A tag attached to each external route. This is not used by the OSPF
protocol. It may be used to manage external routes.
• NSSA external LSA
An NSSA external LSA originates from the ASBR in an NSSA, and is flooded in the NSSA area
only. It has the same format as the AS external LSA.
71 Figure 36 NSSA external LSA format Supported features Multi-process This feature allows multiple OSPF processes to run on a router both simultan eously and independently. Routing information interactions between different processes simulate interactions between different routing protocols. Multiple OSPF processes can use the same RID. An interface of a router can only belong to a single OSPF process. Authentication OSPF can authenticate OSPF packets. Only packets that pass the authentication are received. If an incoming hello packet cannot pass authentication, th e neighbor relationship cannot be established. The authentication type for interfaces attached to a single area must be identical. Authentication types include non-authentication, plaintext authentication, and MD5 ciphertext authentication. The authentication password for interfaces that are at tached to a network segment must be identical. OSPF Graceful Restart Graceful Restart (GR) ensures the continuity of packet forwarding when a routing protocol restarts or an active/standby switchover occurs: • GR Restarter —Graceful restarting router. It must have GR capability. • GR Helper —A neighbor of the GR Restarter. It helps the GR Restarter to complete the GR process. After an OSPF GR Restarter restarts, it must perform the following tasks. • Obtain OSPF neighbor information. • Obtain the LSDB. Before restart, the GR Restarter negotiates GR capabi lity with GR Helpers. During the restart of the GR Restarter, GR Helpers still advertise their adjacencies with the GR Restarter. After restart, the GR Restarter sends GR Helpers an OSPF GR signal so that the GR Helpers do not reset their neighbor relationships with the GR Restarter. Upon receiv ing responses from neighbors, the GR Restarter creates the neighbor relationships. After that, the GR Restarter synchronizes the LSDB with GR-capable neighbors, updates its routing table and forwarding table, and removes stale routes.