Data Center LAN Migration Guide - Juniper Networks
Data Center LAN Migration Guide - Juniper Networks
Data Center LAN Migration Guide - Juniper Networks
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OSI Layer 3: Network Troubleshooting<br />
<strong>Data</strong> <strong>Center</strong> <strong>LAN</strong> <strong>Migration</strong> <strong>Guide</strong><br />
While L1 and L2 problems have limited effect and are local, L3 or routing issues may affect other networks by<br />
propagation and may have a global impact. In the data center, the aggregation/core tiers may be affected. The<br />
following focuses on the operation of OSPF and BGP as they are commonly implemented in data centers to exchange<br />
internal and external routing information.<br />
In a next -generation or newly deployed network, OSPF’s primary responsibility typically is to discover endpoints for internal<br />
BGP. Unlike OSPF, BGP may play multiple roles that include providing connectivity to an external network, information<br />
exchange between VRFs for L3 MPLS VPN or VPLS, eventually carrying data centers internal routes to access routers.<br />
OSPF<br />
A common problem in OSPF is troubleshooting adjacency issues which can occur for multiple reasons: mismatched IP<br />
subnet/mask, area number, area type, authentication, hello/dead interval, network type, or mismatched IP MTU.<br />
The following are useful commands for troubleshooting an OSPF problem:<br />
• show ospf neighbor displays information about OSPF neighbors and the state of the adjacencies which must be<br />
shown as “full.”<br />
• show ospf interface displays information about the status of OSPF interfaces.<br />
• show ospf log logs shortest-path-first (SPF) calculation.<br />
• show ospf statistics displays number and type of OSPF packets sent and received.<br />
• show ospf databases displays entries in the OSPF link-state database (LSDB).<br />
OSPF traceoptions provide the primary debugging tool, and the OSPF operation can be flagged to log error packets<br />
and state transitions along with the events causing them.<br />
BGP<br />
show bgp summary is a primary command used to verify the state of BGP peer sessions, and it should display that the<br />
peering is “established” to be fully operational.<br />
BGP has multiprotocol capabilities made possible through simple extensions that add new address families. This<br />
command also helps to verify which address families are carried over the BGP session, for example, inet-vpn if L3 MPLS<br />
VPN service is required or L2VPN for VPLS.<br />
BGP is a policy-driven routing protocol. It offers flexibility and granularity when implementing routing policy for path<br />
determination and for prefix filtering. A network operator must be familiar with the rich set of attributes that can<br />
be modified and also with the BGP route selection process. Routing policy controls and filters can modify routing<br />
information entering or leaving the router in order to alter forwarding and routing decisions based on the following criteria:<br />
• What should be learned about the network from all protocols?<br />
• What routes should be shared with other routing protocols?<br />
• What should be advertised to other routers?<br />
• What routing information should be modified, if any?<br />
Consistent policies must be applied across the entire network to filter/advertise routes and modify BGP route<br />
attributes. The following commands assist in the troubleshooting of routing policies:<br />
• show route receive-protocol bgp displays received attributes.<br />
• show route advertising-protocol bgp displays route and attributes sent by BGP to a specific peer.<br />
• show route hidden extensive displays routes not usable due to BGP next-hop problems and routes filtered by an<br />
inbound route filter.<br />
Logging of peer state transitions and flagging BGP operations provides a good source of information when investigating<br />
BGP problems.<br />
Copyright © 2012, <strong>Juniper</strong> <strong>Networks</strong>, Inc. 59