Data Center LAN Migration Guide - Juniper Networks

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