Identifying Neighbors and Defining Peer Groups
BGP does not promiscuously attach to other routers—neighbors must be predefined. The neighbor command is used to identify each neighbor and its AS. If the neighbor AS is the same as this router's AS then this is an iBGP connection. If the AS is different, this is an external neighbor. After the neighbor is defined, further neighbor commands are used to describe the policy used to filter routes to the neighbor. Router(config-router)#neighbor ip-address remote-as auto..
Becoming Neighbors: The Hello Process
Hello messages perform three major functions: ■ Discover other OSPF-speaking routers on common subnets ■ Check for agreement on some configuration parameters ■ Monitor health of the neighbors to react if the neighbor fails To discover neighbors, Cisco OSPF routers listen for multicast Hello messages sent to 224.0.0.5— the All OSPF Routers multicast address—on any interfaces that have been enabled for OSPF. The Hellos are sourced from that rout..
There is very little overhead in establishing a peer group, even with only a single member. This approach makes it very easy to add members with identical outgoing policy to the group at a later stage. You may consider defining a peer group for all neighbors, even if the peer group membership consists of only one neighbor. A description can be added to a peer group or an individual neighbor using the neighbor { ip- address | peer-group-name } description textcom..
Two Key Spanning-Tree Protocol Concepts
Spanning Tree calculations make extensive use of two key concepts when creating a loop-free logical topology: • Bridge ID (BID) • Path Cost Bridge IDs A Bridge ID (BID) is a single, 8-byte field that is composed of two subfields as illustrated in Figure 6-5. Figure 6-5. The Bridge ID (BID) Is Composed of Bridge Priority and a MAC Address The low-order subfield consists of a 6-byte MAC address assigned to the switch. The Catal..
Restricting Networks Advertised to a BGP Peer
There are three ways to filter routes in BGP. The first one uses extended access lists and route maps, as follows: Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#access-list 105 deny ip host 172.25.0.0 host 255.255.0.0 Router1(config)#access-list 105 permit ip any any Router1(config)#route-map ACL-RT-FILTER permit 10 Router1(config-route-map)#match ip address 105 Router1(config-route-map)#exit Router1(config)#ro..
