Huawei S6700 Series Configuration Manual page 261

S6700 Series Ethernet Switches
Configuration Guide - IP Routing
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IS-IS Three-Way Handshake
A reliable link layer protocol is required when IS-IS runs on a point-to-point (P2P) link. Based
on ISO 10589, the two-way handshake mechanism of IS-IS uses Hello packets to set up P2P
adjacencies between neighboring switchs. Once the switch receives a Hello packet from its peer,
it regards the status of the peer as Up and sets up an adjacency with the peer.
This mechanism has obvious defects. For example, when an adjacency is set up, the unstable
link status causes the loss of Complete Sequence Number Packets (CSNPs). As a result, the
LSDB fails to be synchronized during the update period of an LSP. If two or more links exist
between two switchs, an adjacency can still be set up when one link is Down and the other is
Up in the same direction. The parameters of the other link, however, are also used in SPF
calculation. Theswitch does not detect any fault of the link that is in the Down state and still
tries to forward packets over this link.
The three-way handshake mechanism addresses the problem on the unreliable P2P link. In three-
way handshake mode, the switch regards the neighbor as Up only after confirming that the
neighbor receives the packet that it sends and then sets up an adjacency with the neighbor. In
addition, a 32-bit circuit ID is used in the three-way handshake mechanism, which is an extension
of the local 8-bit circuit ID that defines 255 P2P links.
Issue 01 (2012-03-15)
Static BFD
To configure static BFD, use command lines to configure single-hop BFD parameters, such
as local and remote discriminators. Then configure the device to send BFD session setup
requests.
A static BFD session can only be established and released manually. A configuration error
will lead to a BFD failure. For example, if the configured local discriminator or remote
discriminator is incorrect, a BFD session will not work properly.
The S6700 supports static IPv4 BFD for IS-IS.
Dynamic BFD
Dynamic BFD refers to the dynamic establishment of BFD sessions using routing protocols.
When a new IS-IS neighbor relationship is set up, BFD is notified of the parameters of the
neighbor and the detection parameters (including source and destination IP addresses).
Then a BFD session will be established based on the received parameters of the neighbor.
Dynamic BFD is more flexible than static BFD.
Connection status between an IS-IS device and its neighbors can be monitored by
exchanging Hello packets at intervals. The sending interval is usually set to 10s, and a
neighbor is declared Down after at least three intervals (during which no response Hello
packet is received from the neighbor). It takes IS-IS some seconds to sense a Down
neighbor, resulting in loss of a large amount of high-speed data.
Dynamic BFD can provide link failure detection with light load and high speed (at the
millisecond level). Dynamic BFD does not take the place of the Hello mechanism of IS-
IS, but helps IS-IS to detect the faults on neighbors or links more quickly, and instruct IS-
IS to recalculate routes to correctly guide packet forwarding.
The S6700 supports dynamic IPv4 and IPv6 BFD for IS-IS.
NOTE
For details about IS-IS GR, see the "IS-IS" chapter in the S6700 Series Ethernet Switches Feature
Description-IP Routing.
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